Add isolatedSymbol parameter to GetMaxBorrowableService (#231)

* Add isolatedSymbol parameter to GetMaxBorrowableService

The isolatedSymbol parameter is required to get max borrow amount
of an isolated margin account.

* Check if isolatedSymbol is set before adding to request param.

Co-authored-by: Jérémy Pansier <jeremypansier@gmail.com>

Co-authored-by: Jérémy Pansier <jeremypansier@gmail.com>

v2/margin_service.go

@@ -729,8 +729,9 @@ func (s *ListMarginTradesService) Do(ctx context.Context, opts ...RequestOption)
 
 // GetMaxBorrowableService get max borrowable of asset
 type GetMaxBorrowableService struct {
-	c     *Client
-	asset string
+	c              *Client
+	asset          string
+	isolatedSymbol string
 }
 
 // Asset set asset
@@ -739,6 +740,12 @@ func (s *GetMaxBorrowableService) Asset(asset string) *GetMaxBorrowableService {
 	return s
 }
 
+// IsolatedSymbol set isolatedSymbol
+func (s *GetMaxBorrowableService) IsolatedSymbol(isolatedSymbol string) *GetMaxBorrowableService {
+	s.isolatedSymbol = isolatedSymbol
+	return s
+}
+
 // Do send request
 func (s *GetMaxBorrowableService) Do(ctx context.Context, opts ...RequestOption) (res *MaxBorrowable, err error) {
 	r := &request{
@@ -747,6 +754,9 @@ func (s *GetMaxBorrowableService) Do(ctx context.Context, opts ...RequestOption)
 		secType:  secTypeSigned,
 	}
 	r.setParam("asset", s.asset)
+	if s.isolatedSymbol != "" {
+		r.setParam("isolatedSymbol", s.isolatedSymbol)
+	}
 	data, err := s.c.callAPI(ctx, r, opts...)
 	if err != nil {
 		return nil, err